Fans were left horrified after Harry Styles was seen choking on water before collapsing to the ground during his sold-out Wembley Stadium performance on Friday, local time.In footage posted to social media, the British singer was seen blowing water out of his mouth for his famous “whale spit” before clutching at his chest and falling backwards.While on the ground, he was seen heaving and attempting to loosen his tie as unaware fans continue to chant “Kiwi” – a reference to the star’s popular song.After 30 seconds, the singer was seen getting to his feet and running across the stage, before resuming the show.Fans took to social media to express their concern for the former One Direction star, branding the ordeal “terrifying”.“I was looking around in pit super confused because I was seeing him from the back. [I was] genuinely concerned,” wrote one fan on X.”When he fell to his knees, my heart stopped in shock. Harry is incredible, it surely takes immense strength and stamina to perform in these temperatures, and yet he always gives it his all,” said another. ️“I nearly had a heart attack watching the live stream! Thought he collapsed from heat or exhaustion! Love you dearly Harry!” wrote a third.“I hate this, it’s so scary,” said another.Another said it was “genuinely very dystopian how he’s on the floor choking and coughing (and you still don’t know if he’s joking around or not) while everyone is filming and chanting kiwi”.The shock sight comes as the UK swelters through a heatwave, with temperatures reaching 37.5C in London.The UK’s Health Security Agency has issued a red health alert, warning there is a “risk to life for even the healthy population”.Temperatures were forecast to reach 33C in Wembley at 5pm when the doors opened and were expected to remain in the low 30s as Styles took to the stage at 8.05pm, following support act Shania Twain. Styles was seen arriving on stage for the latest date of his Together Together tour dressed in shorts, a blazer and an orange shirt.He was heard urging the 80,000 fans present to “look after each other”.“We’re going to look after each other, please try and stay hydrated. If you need anything at any point, please let me know, we can stop at any time,” he said.“It’s all good. We’re gonna look after each other, have fun, dance, get sweaty, sing, scream if you wanna go faster.”Wembley Stadium relaxed its water bottle policy earlier in the week to allow concertgoers to bring metal or hard plastic water bottles inside.They also slashed water prices and offered free sunscreen at information stations.The heatwave has caused havoc across Western Europe over the last week, with authorities saying the record-breaking weather has claimed hundreds of lives.London Ambulance Service chief executive Jason Killens said paramedics responded to the highest-ever number of “life-threatening emergencies” on Wednesday “driven by the extreme heat”.Emergency services responded to a record 642 Category 1 calls that day, which include the most serious, life-threatening injuries and illnesses, such as cardiac arrests and patients who are not breathing.In Spain, where new temperature records have been set for June, the MoMo mortality-rate monitoring system said that 212 deaths between Sunday and Wednesday could be linked to the heat.In Paris, 25 cardiac arrests were recorded over 24 hours on Wednesday, compared with fewer than 10 usually, Health Minister Stephanie Rist’s office said.Since the end of last week, more than 20 people across Germany have also lost their lives in swimming-related accidents, according to the German Life Saving Association.